1. Field of Invention
This invention pertains to the problem of the buildup of radioactive material on the wetted, interior surfaces of the primary cooling systems of water-cooled nuclear power reactors. These radiation fields are a major source of the radiation exposure to plant operating and maintenance personnel.
The buildup of radioactive material is due to the deposition of radioactive corrosion products on the piping surfaces. Although the internal corrosion of the primary coolant systems is usually insignificant from a structural standpoint, trace quantities of corrosion products are transported by the circulating coolant to the reactor core where they are deposited as a thin oxide film on the exterior surfaces of the metallic fuel rods. There, the corrosion products become radioactive as they are exposed to the intense neutron flux of the reactor core. Erosion, spalling, and dissolution of this oxide film return the radioactive corrosion products to the circulating coolant. They are subsequently deposited on and incorporated in the corrosion film of oxides on the interior surfaces of the cooling system piping outside the reactor vessel. Typically, the indigenous corrosion film contains greater than 85 percent of the radioisotopes on the out-of-core surfaces; thus, incorporation in the indigenous corrosion film controls the long-term build-up levels.
2. Description of the Prior Art
The nuclear power industry has been conducting an extensive program of research, analysis, and radiation and chemical measurements to develop a fundamental understanding of the chemical, physical, and nuclear processes that comprise this radiation build-up cycle (1-3)*. The literature prior to 1973 reviewed by Berry and Diegle (4), and the results of a major program sponsored by the Edison Electric Power Institute was described by Anstine (5). These studies have been concerned with: FNT *References to the prior art are listed in the end of this application
1. The corrosion of the cooling system interior surfaces.
2. The release and transport of corrosion products to the core.
3. The deposition, activation, and release from the fuel surfaces.
4. The transport to and deposition on the piping surfaces.
The intent has been to use this fundamental understanding of the process mechanisms to devise new plant operating conditions, piping materials, or coolant chemistries that will reduce and possibly even invert the radiation build-up rates.
A major obstacle to the development of a complete understanding of these fundamental phenomena has been the absence of a definitive, short-term test for how significant the long-term radiation buildup will be under a specific set of laboratory or plant conditions. By analogy, the house paint manufacturers would have a difficult time improving the weather resistance of their formulations if they had to wait for years of weathering to assess the effect of a proposed ingredient.
Lacking a reliable short-term measurement, the investigators of nuclear plant radiation buildup have been forced to compare massive amounts of scattered plant data with the limited results of a few laboratory studies. The plant data have been gathered under widely varying measurement, plant, and materials conditions and have covered many years of plant operation. Although the laboratory conditions have been more controlled, it has been difficult to make them representative of the plant conditions. Also the laboratory tests are usually conducted for a duration of no more than a few months. Correlations between the parameters that could affect build-up rates are difficult to find in the maze of plant data (6), and any proposed fixes resulting from the laboratory work have had to be implemented without waiting for the results of long-term plant verification testing.
There is general agreement that very little corrosion product transport and radiation buildup occurs when a power plant is not operating. Therefore, it is convenient to consider effective full-power time rather than calendar time when these processes are examined. FIG. 1 shows the buildup of the dose rates from the recirculation system piping at boiling water reactors (1).
The exact cause of the large variation among plants remains a major unsolved problem. If the data points for each plant are connected by lines, the dose rates in over 90 percent of the plants increase for the first 4 to 5 effective full-power years and then level off (3,6).
A review of the data indicated to me there are five key data correlations that provide an insight into a technique for assessing radiation build-up effects. These are presented in graphical form in FIGS. 2 through 6.
The first data correlation is shown in FIG. 2. For those plants where radiochemical measurements of the concentrations of isotopes in the cooling water have been published, the leveled-off (equilibrium) dose rate is plotted versus the average soluble Co-60 concentration in the cooling water. The datum for Hatch-2 was taken from reference 7 and for Dresden-2 from reference 8; all other data are from reference 6. This correlation indicates the long-term dose rate is approximately proportional to the Co-60 concentration in the water. This is to be expected, since Co-60 is the dominant gamma-emitting isotope contributing the dose rates from the coolant pipes. Unfortunately, the data are badly scattered. This scatter indicates the long-term dose rates may depend on the cooling water Co-60 concentration, but they are significantly impacted by other parameters that vary between plants.
The second data correlation is shown in FIG. 3. For those plants where significant data have been taken, the leveled-off (equilibrium) dose rate is plotted verses the dose rate at the end of the first fuel cycle (about 1.23 effective full-power years for U.S. boiling water reactors and 0.8 effective full-power year for foreign boiling water reactors). This correlation indicates that the long-term (equilibrium) dose rate is determined primarily during the first fuel cycle (6). At Monticello and Tsuruga, atypical events have occurred that offer explanations for the deviations of these two data points.
The third data correlation is graphed in FIG. 4. The data were derived from Co-60 build-up measurements taken from six coupons inserted in an experimental test loop at the Hatch-2 plant (5). The loop provided for the continuous circulation of actual recirculation system water at near reactor temperatures over the stainless steel coupons. Although the coupons are from three different fabrication lots, the minor differences in fabrication processes and alloy compositions would not be expected to have produced the widely different Co-60 build-up rates. However, the Co-60 concentration, pH, and conductivity in the Hatch-2 cooling water varied considerably during the period when these tests were conducted. Clearly, the initial build-up rates (less than 500 hours) and the long-term (6000 hours) activity levels are strongly influenced by the reactor water conditions during the period just after coupon insertion. If the water conditions are different, the buildup will be different.
The fourth data correlation is shown in FIGS. 5 and 6. It was also derived from the Hatch-2 loop data. The graphs compare the Co-60 build-up curves for two of the coupons with the build-up curve of total corrosion products (elemental weight per unit area) on these coupons. It is clear that the activity growth tracks the film growth during the first two months of exposure.
The fifth data correlation I recognized is that the shape and size of the grains or particles of oxide that comprise the corrosion films are related to the dose rates from those films. At Quad Cities-1, Nine Mile Point-1, and Millstone, where the dose rates started high and leveled off high, the film particles were found to be small and irregularly shape. At Shimane, Vermont Yankee, and Brunswick-2, where dose rates were initially lower and leveled off at lower values, the particles were large single crystals with sharp edges and faces. These data are presented in reference 6.
The five data correlations indicate the water chemistry conditions present during the initial formation of the oxide film are extremely important. These conditions determine the film morphology, which controls the long-range incorporation rate. Consequently, the initial corrosion process is of prime interest.
Corrosion is the destruction of a metal by an electrochemical action at its surface. This electrochemical action always consists of two separate electrode reactions that give rise to the flow of electrons. The anodic reaction is the oxidation of metal to form metal ions: EQU Fe.fwdarw.Fe.sup.+2 1/2O.sub.2 e-.
If the water contains dissolved oxygen and has a neutral pH (as in the cooling water of a boiling water reactor) some of the ferrous ion may be oxidized: EQU 2Fe.sup.+2 +1/2O.sub.2 +H.sub.2 O.fwdarw.2FeOH.sup.+2.
The electrons from the anodic reaction flow through the metal to a nearby location where they combine with oxygen and water in the cathodic reaction: EQU 2e-+1/2O.sub.2 +H.sub.2 O.fwdarw.2OH-.
The ferrous and ferric ions combine with the hydroxide ions to form magnetite: EQU 2FeOH.sup.+2 +Fe.sup.+2 +6OH-.fwdarw.Fe.sub.3 O.sub.4 +4H.sub.2 O.
Therefore, three times the sum of the anodic and cathodic reactions plus the ferrous ion oxidation reaction gives the net film formation reaction: EQU 3Fe+2O.sub.2 O.fwdarw.Fe.sub.3 O.sub.4.
These chemical processes are shown schematically in FIG. 7.
Depending on the pH and the relative concentrations of dissolved hydrogen and oxygen in the water, a cathodic reaction involving the reduction of hydrogen ions can also be significant: EQU 2e-+2H+.fwdarw.H.sub.2.
However, the production of an oxide film is still the overall result.
While the absolute potential of the anodic/cathodic reaction cannot be measured directly, this potential can be measured relative to a reference potential created by a reference electrode. The standard hydrogen electrode is, by convention, assigned a potential of zero at all temperatures. However, the standard hydrogen electrode would be difficult to use in some environments, such as the high temperatures and pressures of nuclear reactor cooling systems. A secondary standard electrode, calibrated against the standard hydrogen electrode, is used instead. A silver/silver chloride electrode, enclosed in a Teflon chamber filled with a 0.01 molal potassium chloride solution saturated with silver chloride, serves well for this purpose. The liquid junction is an asbestos wick. The standard potential for such a Ag/AgCl electrode has been determined by Greeley (9) for temperatures up to 300.degree. C. Loss of KCl through the liquid junction and the increasing AgCl solubility with increasing temperature can be problems for long-term electrochemical potential measurements but not for short-duration electrochemical potential measurements.
The driving force for corrosion is the electrochemical potential difference from the metal surface to the solution. A potential drop occurs across the metal/oxide interface, through the oxide, and across the oxide/solution interface. To maintain the electric field, as the film grows, the potential must also grow. Therefore, the electrochemical potential of the measuring electrode tracks the growth of the oxide film on that electrode.
In all previous applications of electrochemical potential measurements to nuclear reactor coolant systems, the interest has been in electrochemical potential the corrosion film that forms on the measuring electrode is well established and growing very slowly. Then the electrochemical potential indicated by the device will be unaffected by the characteristics of the growing film over the duration of the measurements. The electrochemical potential measuring and reference electrodes have been allowed to remain in the coolant for several weeks or months before measurements are made to assure the measuring electrode is adequately prefilmed.
It is an object of this invention to make electrochemical potential measurements of the coolant of a nuclear reactor with an unprefilmed measuring electrode so the measurements will be affected by the characteristics of the growing film.
It is another object of this invention to provide a method for reliably predicting what the long-term radiation levels of nuclear reactor coolant piping will be if the coolant conditions are sustained.
Other objects and advantages of this invention will become apparent to a person skilled in the art from a reading of the accompanying drawings described immediately hereafter, the description of the invention, and the appended claims. It is to be understood, however, that the invention is not limited to the embodiment illustrated and described since it may be embodied in various forms within the scope of the appended claims.